Amid recent concerns about peak oil, urgent issues have arisen, such as the shift to bio-based technologies for not only energy but also chemical refineries, and it became a critical issue that the conversion of industrial starting materials from petroleum to biomass.
In humans, 3-hydroxybutyric acid, which is produced from acetyl CoA in the liver, is used as an energy source for the brain when blood glucose levels are low. Further, it can prevent the transfer of enterobacteria into the blood (PTL 1). Therefore, 3-hydroxybutyric acid is used in infusions. This compound is also used as a starting material for biodegradable plastics.
Referring to the “Hopeful 32 Starting Materials for Biorefineries” shown in NPL 1, 3-hydroxybutyric acid is mentioned as a promising compound as a starting or intermediate material for biorefineries. Further expanded use of this compound is highly expected.
Known processes for producing 3-hydroxybutyric acid are as follows. For example, since this compound is a monomer of poly-3-hydroxybutyrate (hereinafter also referred to as “PHB” in the present specification), PHB is produced by various bacterial cells, and then degraded by lipases, etc., which have been prepared separately, to thereby obtain 3-hydroxybutyric acid, which is a monomer of PHB (PTL 2). NPL 2 shows a process in which 8.7 g/L of 3-hydroxybutyric acid is obtained by using mutants, and NPL 3 shows a process in which 3-hydroxybutyric acid is obtained with a yield of 12 g/L by using a gene recombination technique.
The present inventor examined an efficient process of culturing Spirulina microalgae, which is known to have little contamination by other bacteria during commercial outdoor incubation, and found that specific halophilic bacterium, grew under certain conditions as the only contaminating bacterium. Since in general the halophilic bacteria generally grew well in a medium with a pH of about 5 to 12 containing a high concentration of sodium, it was presumed that contamination by other bacteria hardly occurred, even under aerobic fermentation. Then, examination of the assimilation of various carbon sources by the halophilic bacteria revealed that a remarkable amount of polyhydroxyalkanoates (PHAs) was accumulated in the cells of the halophilic bacteria (PTL 3).
Moreover, PTL 4 shows studies specific to the production of PHAs by the halophilic bacteria, and the halophilic bacteria are known to be involved in the production of specific substances, such as lactic acid and acetic acid (PTL 4). Furthermore, NPL 6 reports a process for producing 3-hydroxybutyric acid with a yield of 117 g/L by culturing bacterial cells belonging to a specific genus, and then subjecting the bacterial cells to autolysis under anaerobic conditions for about 6 hours.